A Laser Photolysis Shock Tube Study of the Reaction of OH with NH₃

Abstract

The reaction of OH with NH₃ has been studied in reflected shock wave experiments using laser photolysis of NH₃/N₂O/Ar mixtures. Quantitative time-histories of the OH(X ²Π_i) radical were measured behind the shock waves using cw, narrow-line width laser absorption at 307 nm. OH was generated using post-shock laser photolysis of ammonia followed by the reaction of atomic hydrogen with N₂O:

$$N{H_3} + \hbar \nu (193nm) \to N{H_2} + H{N_2}O + H \to {N_2} + OH$$

((1))

Ammonia photolysis yields were confirmed by quantitative measurements of the NH₂ radical using cw, narrow-linewidth laser absorption at 597 nm. Initial mixture concentrations and test conditions were chosen such that photolysis and pyrolysis of N₂O and pyrolysis of NH₃ do not play a significant role in OH reaction kinetics. Following the production of OH by reaction (1), OH-removal is dominated by reaction (2):

$$N{H_3} + OH \to N{H_2} + {H_2}O$$

((2))

NH₃ + OH → NH₂ + H₂O

This makes it possible to determine the second-order rate coefficient of reaction (2) by adjusting the value of k ₂ in a detailed reaction mechanism until calculated OH concentrations fit the measured OH profiles. A least-squares two-parameter fit of the results is given by:

$${k_2} = 1.1 \times {10^{14}}\exp ( - 4600/T,K)c{m^3}mol{e^{ - 1}}{s^{ - 1}}(f = 0.65,F = 1.45)T = 1240 - 1480K,$$

where f and F are the minimum and maximum rate coefficient factors. The activation energy of the above expression is a parameter determined by the least-squares fitting procedure, and is subject to large uncertainties.

This result is in excellent agreement with the expression recommended by Cohen and Westberg (1991) in their review of reaction (2):

$${k_2} = 5.0 \times {10^7}{T^{1.6}}\exp ( - 480/T,K)c{m^3}mol{e^{ - 1}}{s^{ - 1}}T = 225 - 3000K.$$